Combustion in engine cylinders may be affected by ambient humidity. Spark ignited internal combustion engines may encounter undesired combustion events, such as knock, during low humidity conditions when the combustion rate may increase. During higher humidity conditions, if the engine does not account for diluent composition in the air fuel mixture, water in charge air may act as a diluent and the combustion rate may decrease causing a reduction in engine efficiency.
Various approaches are provided to compensate for ambient humidity in order to improve combustion stability and engine efficiency. In one example, as shown in US 20140109870, Glugla et al. teaches adjustments to spark timing based on ambient humidity and condensate level in a charge air cooler. During conditions when condensate is released from the charge air cooler, spark timing may be adjusted based on the resulting intake manifold humidity and the current ambient humidity. For example, spark timing may be advanced from a timing that is retarded from MBT to or towards MBT. By applying the spark advance, knock toleration and combustion stability may be improved during the condensate purging.
However, the inventors herein have recognized potential issues with such an approach. As one example, by adjusting spark timing solely based on intake humidity and ambient humidity levels, throttle response and combustion stability may be adversely affected during tip-in conditions. In particular, spark timing may not be sufficiently advanced. Therefore, even after adjusting spark timing based on humidity, knock may be encountered due to transient changes in air charge response during a tip-in. In addition, the time to torque response on the tip-in may be degraded due to a portion of charge air being replaced by humidity.
In one example, the issues described above may be addressed by a method for an engine comprising: adjusting spark timing applied at an operator tip-in event based on each of ambient humidity and transient air charge parameters of the tip-in event. In this way, by continually adjusting spark timing based on each of ambient humidity and air charge parameters, combustion stability and throttle response may be improved.
In one example, an ambient humidity may be estimated based on inputs from one or more engine sensors, such as a humidity sensor. During steady-state engine operation, spark timing may be adjusted relative to maximum brake torque (MBT) based on ambient humidity, spark timing retarded from MBT as ambient humidity decreases. During transient engine operation, particularly during a tip-in event when there is an increase in operator torque demand, spark timing may be adjusted based on the ambient humidity and further based on the transient air charge parameters (e.g., tip-in parameters). These may include, for example, a rate of change of air charge and a peak air charge of the tip-in. A three-dimensional map plotted a function of each of the ambient humidity, the rate of change of air charge, and the peak air charge may be used to determine the spark timing adjustment to apply during the tip-in condition. The spark timing adjustment to be applied at the tip-in may include each of an amount of spark timing retard from MBT, a duration of applying the spark timing retard (e.g., number of combustion events or engine cycles or which to apply the spark retard), and a rate of change of spark timing retard, as determined using the three-dimensional map or an alternate algorithm. As an example, spark timing may be retarded further from MBT as one or more of a rate of change of the air charge and a peak air charge of the tip-in increases. As another example, a higher amount of spark timing retard may be applied at lower humidity levels. During a subsequent tip-out, the spark timing adjustment may be performed based only on the humidity.
The inventors have recognized that spark timing adjustments that are based on humidity and tip-in air charge parameters cannot be provided by simply compounding a first spark timing adjustment based on humidity at the time of the tip-in with a second spark timing adjustment based on the tip-in air charge parameters. This is because of the synergistic interactions of the humidity based first spark timing adjustment with the tip-in charge parameters based first spark timing adjustment over at least a portion of the tip-in. If the spark retard applied during this portion of the tip-in was merely the sum of the first and second spark timing adjustments, more spark retard than is necessary may be applied, resulting in a drop in fuel economy. Likewise, there may be portions of the tip-in where the first spark timing adjustment that compensates for the humidity effect works against the second spark timing adjustment that compensates for the charge flow effect. If the spark retard applied during this portion of the tip-in was merely the sum of the first and second spark timing adjustments, less spark retard than is necessary may be applied, resulting in tip-in knock. By adjusting an amount of spark retard and a rate of change of the spark retard over the duration of the tip-in based on the humidity, the rate of change of air flow, and the peak air flow during the tip-in, a more accurate spark timing profile that improves tip-in response and reduces knock occurrence can be provided.
In this way, by adjusting an amount of spark timing retard during a tip-in based on each of humidity and transient air charge response, time to torque may be expedited and throttle response on a tip-in may be improved. Further, by increasing the amount of spark timing retard during lower humidity conditions, combustion stability may be improved and possibility of knock may be decreased after a tip-in. By decreasing the amount of spark timing retard during higher humidity conditions, combustion rate may be improved and engine efficiency may be increased. The technical effect of using a three-dimensional map to determine spark timing during tip-in conditions is that in addition to considering the effect of dilution due to humidity, transient changes in air charge parameters may be accounted for while determining an optimal spark timing during a tip-in. In this way, a spark timing may be effectively adjusted during a tip-in to improve overall combustion stability and engine operation.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.